In the letter, Comey said "the FBI cannot yet assess" whether the information is "significant" nor could he offer a timetable for how long it will take investigators to make an assessment.

Comey wrote that the discovery of the additional messages had occurred "in connection with an unrelated case."

In July, Comey announced that while Clinton and her aides while she was secretary of State had been "extremely careless" in the way they'd handled classified information, he recommended that no criminal charges be filed.

Soon after, the director testified before skeptical GOP lawmakers to explain the bureau's recommendation, which had been adopted by Attorney General Loretta Lynch.

“We’re mystified and confused by the fact pattern you laid out and the conclusion you reached," House Oversight Committee Chairman Jason Chaffetz, R-Utah, told Comey.

Comey, however, was unequivocal in maintaining that the conclusions of investigators were not a close call.

“There is no way anybody would bring a case against John Doe or Hillary Clinton for the second time in 100 years based on those facts," he told the House panel on July 7.

The news of the FBI's new inquiry comes just 11 days before Americans go to the polls in an election where Clinton had opened up a significant lead against Republican Donald Trump.

Trump has cited the closed FBI inquiry as evidence that the election was "rigged" against him, and at a recent debate said that, if he's elected president, he would appoint a special prosecutor to investigate Clinton.